A typical plant for production of gasoline, liquid hydrocarbons such as diesel, or methanol from for example natural gas typically comprises of the following main process units: (a) air separation, (b) synthesis gas preparation via ATR, (c) the actual synthesis of e.g. diesel, (d) upgrading and/or separation. In the synthesis gas preparation section, hydrocarbon feedstock, normally natural gas is normally pre-reformed, and then passed through an autothermal reformer (ATR) to produce a synthesis gas. An oxygen containing stream is also added to the ATR. This synthesis gas is cooled, water is removed and the thus dehydrated synthesis gas is converted to a raw product. The raw product is then upgraded and/or separated from undesired by-products to provide the desired end product, such as diesel or gasoline.
As an example, in a plant for production of diesel, Fischer-Tropsch (FT) synthesis is carried out for producing a mixture of hydrocarbons comprising for example wax and liquids as well as a range of lighter and gaseous hydrocarbons with hydrogen and carbon monoxide as reactants. In this case the upgrading section would normally comprise hydrocracking for production of the final product which is mainly diesel.
The FT synthesis often also produces an off-gas in the form of so-called tail gas comprising unreacted hydrogen and carbon monoxide and light hydrocarbons (typically with 5 or less carbon atoms) including olefins. The tail gas often also comprises carbon dioxide and other typically inert compounds such as nitrogen and argon. Part of the tail gas may be recycled to the ATR section to adjust the H2/CO-molar ratio in the synthesis gas to the desired value for FT synthesis which typically is around 2.
In some FT-synthesis, in particular so-called Low Temperature FT-synthesis utilising catalysts comprising cobalt, carbon dioxide is inert in the synthesis, and it may be advantageous to remove carbon dioxide partly or completely from the synthesis gas used in the FT-process. Another example is a plant for production of methanol. In this case the actual synthesis is production of methanol from the synthesis gas. The upgrading and purification is in often one or a number of distillation columns to produce the methanol in the required purity depending upon the final application.
Typically large scale plants are based on ATR as described above for the production of synthesis gas. A low steam-to-carbon molar ratio is preferred to obtain the highest plant efficiency (energy efficiency) and the lowest overall capital cost.
An alternative to ATR is to use a synthesis gas unit based on steam reforming (SMR). However, for large scale plants, synthesis gas units based on SMR are known to be less efficient than ATR and also more capital intensive.
Large scale plants are expensive and there is therefore a huge incentive to improving the plant efficiency. One well known method of improving the plant efficiency is to combine ATR with heat exchange reforming (HER). An HER reactor may be installed upstream and in series with the ATR, or in parallel to the ATR. In both cases the effluent from the ATR is used as source for the heat needed for the endothermic steam reforming reaction taking place in the HER. However, the effluent leaving the ATR is rich in carbon monoxide due to the desired operation at low steam-to-carbon molar ratio in the ATR, which is normally below 1.2, e.g. below 1.0 or below 0.8. Such gases may—when they are contacted with metal surfaces in a certain temperature window—lead to so-called metal dusting corrosion of such surfaces of, in this case, the heat exchange reformer.
Metal dusting shall not be confused with carbon deposition. The latter is a phenomenon in which carbon deposits in catalysts and cause their deactivation and/or a rapid increase of the pressure drop to high levels. Known measures to mitigate this problem have been saturating hydrocarbons in the gas fed to the reformer, as well as using noble catalysts as reforming catalysts.
Unsaturated hydrocarbons are easy to crack and therefore end as carbon deposits in catalysts. In contrast, metal dusting is a completely different phenomenon which involves metal disintegrating to dust. It has to be understood also that carbon deposition on catalysts or on metal surfaces does not necessarily lead to metal dusting. Metal dusting is a deteriorating attack of the carbon monoxide rich gas on alloys based on iron and/or nickel and conventional ways of protecting against metal dusting have been the use of expensive high alloy steels.
The operation of autothermal reforming and heat exchange reforming in parallel or series is well known in the art. For instance, WO 2012/084135 (FIG. 4 herein) shows a series arrangement in which an ATR effluent which together with steam enters the non-catalytic side (shell side) of a heat exchange reformer. Metal dusting in the heat exchange reformer is thus mitigated by adding steam to the ATR effluent gas. Being a series arrangement, such ATR effluent gas is not combined with primary reformed gas from the heat exchange reformer before delivering heat to the reforming reactions in the heat exchange reformer. Instead the gas leaving the catalyst side of the heat exchange reformer is directed to the autothermal reformer.
WO-A-2013/189791 shows also a series arrangement in which a portion of the ATR effluent gas is used to deliver heat in the heat exchange reformer while another portion is by-passed and passed through a waste heat boiler. This in order to solve the problem of temperature control in the heat exchange reformer, for example when fouling occurs. Metal dusting issues are not addressed.
EP-A-1403216 (FIG. 2) and EP-A-1106570 show parallel arrangements of heat exchange reforming and autothermal reforming in which all ATR effluent gas is combined with reformed gas from the heat exchange reformer and then used to deliver heat to the reforming reactions in the heat exchange reformer.
As mentioned above, it is an advantage in plant design to reduce the steam-to-carbon molar ratio to optimise plant economics, as i.a. less water is carried in the process. However, when the steam-to-carbon molar ratio in the ATR is reduced, particularly to values below 1.0, the “agressivity” of the ATR effluent gas increases, meaning that its metal dusting potential increases. Therefore the combination of ATR operating at a low steam-to-carbon molar ratio and heat exchange reforming is very challenging.
Metal dusting is a complex process involving many steps. The potential of a gas to cause metal dusting if often evaluated considering one or both of the following reactions:CO+H2C+H2O  (a)2COC+CO2  (b)
The reaction quotient Q can for the two reaction be expressed as:Qa=PH2O/(PCO×PH2)  Reaction (a)Qb=PCO2/(PCO×PCO)  Reaction (b)
The thermodynamic potential for metal dusting increases as the value of Q decreases at a given temperature of the metallic surface. In the above formulas Px denotes the partial pressure of component X.
It is therefore an object of the present invention to provide a process combining ATR and HER in parallel for production of synthesis gas leading to an overall higher efficiency of plants for production of products such as methanol, gasoline and diesel from natural gas.
This and other objects are solved by the present invention.